1. Field of the Invention
The invention pertains to the field of polyurethane manufacture and more particularly relates to producing polyurethanes in relatively cold molds using a catalyst additive.
2. Description of Other Relevant Methods in the Field
Polyurethanes are typically prepared by the reaction of a polyisocyanate and a polyol in the presence of a catalyst. The catalyst is employed to promote at least two, and sometimes three, major reactions that must proceed simultaneously and competitively at balanced rates during the process in order to provide polyurethanes with the desired physical characteristics. One reaction is a chain extending isocyanate-hydroxyl reaction by which a hydroxyl-containing molecule is reacted with an isocyanate-containing molecule to form a urethane. This increases the viscosity of the mixture and provides a polyurethane containing a secondary nitrogen atom in the urethane groups. A second reaction is a crosslinking isocyanate urethane reaction by which an isocyanate-containing molecule reacts with a urethane group containing a secondary nitrogen atom. The third reaction which may be involved is an isocyanate-water reaction by which an isocyanate-terminated molecule is extended and by which carbon dioxide is generated to blow or assist in the blowing of the foam. The third reaction is not essential if an extraneous blowing agent, such as a halogenated, normally liquid hydrocarbon, carbon dioxide, etc. is employed, but is essential if all or even a part of the gas for foam generation is to be generated by this in situ reaction (e.g. in the preparation of "one-shot" flexible polyurethane foams).
The reactions must proceed simultaneously at optimum balanced rates relative to each other in order to obtain a good foam structure. If carbon dioxide evolution is too rapid in comparison with chain extension, the foam will collapse. If the chain extension is too rapid in comparison with carbon dioxide evolution, foam rise will be restricted, resulting in a high density foam with a high percentage of poorly defined cells. The foam will not be stable in the absence of adequate crosslinking.
It has long been known that tertiary amines, such as trimethylamine, triethylamine, etc., are effective for catalyzing the second crosslinking reaction. Other typical tertiary amines are set forth in U.S. Pat. Nos. 3,925,368; 3,127,436 and 3,243,387 and German OLS Nos. 2,354,952 and 2,259,980. Some of the tertiary amines are effective for catalyzing the third water-isocyanate reaction for carbon dioxide evolution. However, tertiary amines are only partially effective as catalysts for the first chain extension reaction. To overcome this problem, the so-called "prepolymer" technique has been developed wherein a hydroxy-containing polyol component is partially reacted with the isocyanate component in order to obtain a liquid prepolymer containing free isocyanate groups. This prepolymer is then reacted with additional polyol in the presence of a tertiary amine to provide a foam. This method is still commonly employed in preparing rigid urethane foams, but has proven less satisfactory for the production of flexible urethane foams.
For flexible foams, a one-step or "one-shot" process has been developed wherein a tertiary amine, such as triethylenediamine, is employed in conjunction with an organic tin compound. Triethylenediamine is particularly active for promoting the water-isocyanate reaction and the tin compound is particularly active in synergistic combination with the triethylenediamine for promoting the chain extension reaction. However, even here, the results obtained leave much to be desired. Triethylenediamine is a solid and must be dissolved prior to use to avoid processing difficulties. Also, triethylenediamine and other of the prior art amines can impart a strong amine odor to the polyurethane foam.
In addition to problems of odor and handling due to solid character, other tertiary amines suffer still further deficiencies. For example, in some instances the compounds are relatively high in volatility leading to obvious safety problems. In addition, some catalysts of this type do not provide sufficient delay in foaming, which delay is particularly desirable in molding applications to allow sufficient time to situate the preform mix in the mold. Yet other catalysts, while meeting specifications in this area do not yield foams with a desirable tack-free time.
While certain tertiary amines are somewhat suitable in this catalytic area they nevertheless do not have a sufficiently high tertiary amine content in terms of the number of tertiary amines compared to overall molecular weight. It is believed that the higher the tertiary amine content the more rapid the catalytic activity in the polyurethane art.
Heterocyclic tertiary amines are known as urethane catalysts, the most well known being perhaps N-ethylmorpholine. Unfortunately, this particular catalyst also has a high amine odor which is transferred to resultant urethane foam, which is undesirable. U.S. Pat. No. 3,087,928 contains a brief suggestion that N-alkylmorpholines are useful as catalysts in the production of polyurethane foams. A number of heterocyclic tertiary amines have recently been found to be catalytically active for polyurethane and polyisocyanurate production. For example, U.S. Pat. No. 4,251,637 shows that tertiary amino substituted oxazolidines are useful as polyisocyanurate catalysts. These materials may be prepared by reacting a tertiary-primary diamine with a glycol and then using a formaldehyde treatment. Beta-aminopropionitriles containing both oxygen and nitrogen in their rings have found utility as urethane catalysts according to U.S. Pat. No. 3,925,268. Utility as a polyurethane catalyst is also found for bis-(1,4-beta-amino carbonyl-ethyl)-piperazines according to U.S. Pat. Nos. 4,011,223 and 4,012,445 and for 4-(2-dimethylaminoethyl)morpholine described in U.S. Pat. No. 3,786,005. Dialkylaminoalkylimidazoles are other heterocyclic tertiary amines useful as urethane catalysts as disclosed in U.S. Pat. No. 3,912,689. The compound alkanolaminotriazines and hexahydrotriazines catalyze the creation of carbodiimide and isocyanurate linkages as revealed in U.S. Pat. No. 3,981,829. Other tertiary amines recently found to be useful catalysts which do not contain cyclic portions are described in U.S. Pat. Nos. 4,022,720; 4,026,840; 4,033,911; 4,038,210 and 4,048,107.
Blends of catalysts have also been found to be useful. For example, U.S. Pat. No. 4,326,042 teaches that N-methoxypropylmorpholine, N-butylmorpholine and N,N'-dimethylpiperazine together form a catalyst system which gives a homogeneous activator solution that can provide finer, more uniform cells to polyester-based polyurethane foams than could be had by each catalyst used alone. A similar catalyst system is described in patent application Ser. No. 328,291 filed on Dec. 7, 1981 now U.S. Pat. No. 4,376,832.
One problem with making molded high resilience flexible foams occurs if the mold is too cold. Normally, the mold is at a temperature between 120.degree. and 155.degree. F. If the mold temperature is lower than 120.degree. F., voids or holes start to appear on the surface of the foam. It would be desirable to devise a formulation, catalyst or method which would produce a good foam without voids when the mold temperature drops below 120.degree. F.